Boos are the spirits of dead troops
in Bowser's army. They were modified from their original versions by Bowser's
Magikoopas, and were all given the same form so they would work together
with each other as a race all its own, instead of acting as individuals.

Boos are led by the great King Boo,
who led the kidnapping of Mario in Luigi's Mansion. King Boo was formerly
a thief who made his riches by stealing coins in Mario Party. He used this
wealth to become king.

The Boos were once subservient to Bowser,
but have since broken away and established a society, and have made a living
(or dieing) of stealing, scaring, and pulling mischief.

The Boos currently inhabit regions of
Forever Forest and various Ghost Houses in Dinosaur World.